The Forerunners are a highly advanced alien species from the Halo Universe. Their species was created on planet Ghilbalb by an hyper-advanced species known as the Precursors which were mythologized by them. The Forerunners weren't chosen for the Mantle by the Precursors and they went to war against each other. The Forerunner civilization won and drove them out of the galaxy. Eons of peace lasted until a parasite named The Flood invaded and threatened all life in the Milky Way galaxy. The Forerunners fought a forlorn war against them. They activated vast superweapons named Halos to destroy all life in the galaxy. Before they did this they collected two pairs of every race in the galaxy and left until the parasite was killed. After the Floods were defeated they seeded life again and swore never to return to the galaxy.
They classify as a 3.2 civilization on the Kardashev scale.